Introduce MultisetSemaphore: A concurrency primitive for managing access to at most K unique things at once.

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+// Copyright 2016 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.concurrent;
+
+import com.google.common.collect.ConcurrentHashMultiset;
+import com.google.common.collect.MapMaker;
+import com.google.common.collect.Maps;
+import com.google.common.collect.Multiset;
+import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
+import com.google.devtools.build.lib.util.Preconditions;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Semaphore;
+
+/**
+ * A concurrency primitive for managing access to at most K unique things at once, for a fixed K.
+ *
+ * <p>You can think of this as a pair of a {@link Semaphore} with K total permits and a
+ * {@link Multiset}, with permits being doled out and returned based on the current contents of the
+ * {@link Multiset}.
+ */
+@ThreadSafe
+public abstract class MultisetSemaphore<T> {
+  /**
+   * Blocks until permits are available for all the values in {@code valuesToAcquire}, and then
+   * atomically acquires these permits.
+   *
+   * <p>{@code acquireAll(valuesToAcquire)} atomically does the following
+   * <ol>
+   *   <li>Computes {@code m}, the number of values in {@code valuesToAcquire} that are not
+   *   currently in the backing {@link Multiset}.
+   *   <li>Adds {@code valuesToAcquire} to the backing {@link Multiset}.
+   *   <li>Blocks until {@code m} permits are available from the backing {@link Semaphore}.
+   *   <li>Acquires these permits.
+   * </ol>
+   */
+  public abstract void acquireAll(Set<T> valuesToAcquire) throws InterruptedException;
+
+  /**
+   * Atomically releases permits for all the values in {@code valuesToAcquire}.
+   *
+   * <p>{@code releaseAll(valuesToRelease)} atomically does the following
+   * <ol>
+   *   <li>Computes {@code m}, the number of values in {@code valuesToRelease} that are currently in
+   *   the backing {@link Multiset} with multiplicity 1.
+   *   <li>Removes {@code valuesToRelease} from the backing {@link Multiset}.
+   *   <li>Release {@code m} permits from the backing {@link Semaphore}.
+   * </ol>
+   *
+   * <p>Assumes that this {@link MultisetSemaphore} has already given out permits for all the
+   * values in {@code valuesToAcquire}.
+   */
+  public abstract void releaseAll(Set<T> valuesToRelease);
+
+  /**
+   * Returns a {@link MultisetSemaphore} with a backing {@link Semaphore} that has an unbounded
+   * number of permits; that is, {@link #acquireAll} will never block.
+   */
+  public static <T> MultisetSemaphore<T> unbounded() {
+    return UnboundedMultisetSemaphore.instance();
+  }
+
+  /** Builder for {@link MultisetSemaphore} instances. */
+  public static class Builder {
+    private static final int UNSET_INT = -1;
+
+    private int maxNumUniqueValues = UNSET_INT;
+    private MapMaker mapMaker = new MapMaker();
+
+    private Builder() {
+    }
+
+    /**
+     * Sets the maximum number of unique values for which permits can be held at once in the
+     * to-be-constructed {@link MultisetSemaphore}.
+     */
+    public Builder maxNumUniqueValues(int maxNumUniqueValues) {
+      Preconditions.checkState(
+          maxNumUniqueValues > 0,
+          "maxNumUniqueValues must be positive (was %d)",
+          maxNumUniqueValues);
+      this.maxNumUniqueValues = maxNumUniqueValues;
+      return this;
+    }
+
+    /**
+     * Sets the concurrency level (expected number of concurrent usages) of internal data structures
+     * of the to-be-constructed {@link MultisetSemaphore}.
+     *
+     * <p>This is a hint for tweaking performance and lock contention.
+     */
+    public Builder concurrencyLevel(int concurrencyLevel) {
+      mapMaker = mapMaker.concurrencyLevel(concurrencyLevel);
+      return this;
+    }
+
+    public <T> MultisetSemaphore<T> build() {
+      Preconditions.checkState(
+          maxNumUniqueValues != UNSET_INT,
+          "maxNumUniqueValues(int) must be specified");
+      return new BoundedMultisetSemaphore<>(maxNumUniqueValues, mapMaker);
+    }
+  }
+
+  /** Returns a fresh {@link Builder}. */
+  public static Builder newBuilder() {
+    return new Builder();
+  }
+
+  private static class UnboundedMultisetSemaphore<T> extends MultisetSemaphore<T> {
+    private static final UnboundedMultisetSemaphore<Object> INSTANCE =
+        new UnboundedMultisetSemaphore<Object>();
+
+    private UnboundedMultisetSemaphore() {
+    }
+
+    @SuppressWarnings("unchecked")
+    private static <T> UnboundedMultisetSemaphore<T> instance() {
+      return (UnboundedMultisetSemaphore<T>) INSTANCE;
+    }
+
+    @Override
+    public void acquireAll(Set<T> valuesToAcquire) throws InterruptedException {
+    }
+
+    @Override
+    public void releaseAll(Set<T> valuesToRelease) {
+    }
+  }
+
+  private static class BoundedMultisetSemaphore<T> extends MultisetSemaphore<T> {
+    // Implementation strategy:
+    //
+    // We have a single Semaphore, access to which is managed by two levels of Multisets, the first
+    // of which is an approximate accounting of the current multiplicities, and the second of which
+    // is an accurate accounting of the current multiplicities. The first level is used to decide
+    // how many permits to acquire from the semaphore on acquireAll and the second level is used to
+    // decide how many permits to release from the semaphore on releaseAll. The separation between
+    // these two levels ensure the atomicity of acquireAll and releaseAll.
+
+    // We also have a map of CountDownLatches, used to handle the case where there is a not-empty
+    // set that is a subset of the set of values for which multiple threads are concurrently trying
+    // to acquire permits.
+
+    private final Semaphore semaphore;
+    private final ConcurrentHashMultiset<T> tentativeValues;
+    private final ConcurrentHashMultiset<T> actualValues;
+    private final ConcurrentMap<T, CountDownLatch> latches;
+
+    private BoundedMultisetSemaphore(int maxNumUniqueValues, MapMaker mapMaker) {
+      this.semaphore = new Semaphore(maxNumUniqueValues);
+      // TODO(nharmata): Use ConcurrentHashMultiset#create(ConcurrentMap<E, AtomicInteger>) when
+      // Bazel is switched to use a more recent version of Guava. Until then we'll have unnecessary
+      // contention when using these Multisets.
+      this.tentativeValues = ConcurrentHashMultiset.create();
+      this.actualValues = ConcurrentHashMultiset.create();
+      this.latches = mapMaker.makeMap();
+    }
+
+    @Override
+    public void acquireAll(Set<T> valuesToAcquire) throws InterruptedException {
+      int numValuesToAcquire = valuesToAcquire.size();
+      HashMap<T, CountDownLatch> latchesToCountDownByValue =
+          Maps.newHashMapWithExpectedSize(numValuesToAcquire);
+      ArrayList<CountDownLatch> latchesToAwait = new ArrayList<>(numValuesToAcquire);
+      for (T value : valuesToAcquire) {
+        int oldCount = tentativeValues.add(value, 1);
+        if (oldCount == 0) {
+          // The value was just uniquely added by us.
+          CountDownLatch latch = new CountDownLatch(1);
+          Preconditions.checkState(latches.put(value, latch) == null, value);
+          latchesToCountDownByValue.put(value, latch);
+        } else {
+          CountDownLatch latch = latches.get(value);
+          if (latch != null) {
+            // The value was recently added by another thread, and that thread is still waiting to
+            // acquire a permit for it.
+            latchesToAwait.add(latch);
+          }
+        }
+      }
+
+      int numUniqueValuesToAcquire = latchesToCountDownByValue.size();
+      semaphore.acquire(numUniqueValuesToAcquire);
+      for (T value : valuesToAcquire) {
+        actualValues.add(value);
+      }
+      for (Map.Entry<T, CountDownLatch> entry : latchesToCountDownByValue.entrySet()) {
+        T value = entry.getKey();
+        CountDownLatch latchToCountDown = entry.getValue();
+        latchToCountDown.countDown();
+        Preconditions.checkState(latchToCountDown == latches.remove(value), value);
+      }
+      for (CountDownLatch latchToAwait : latchesToAwait) {
+        latchToAwait.await();
+      }
+    }
+
+    @Override
+    public void releaseAll(Set<T> valuesToRelease) {
+      int numUniqueValuesToRelease = 0;
+      for (T value : valuesToRelease) {
+        int oldCount = actualValues.remove(value, 1);
+        Preconditions.checkState(oldCount >= 0, "%d %s", oldCount, value);
+        if (oldCount == 1) {
+          numUniqueValuesToRelease++;
+        }
+        tentativeValues.remove(value, 1);
+      }
+
+      semaphore.release(numUniqueValuesToRelease);
+    }
+  }
+}